In order to extract motion-related information, a so-called “Edge Motion Detection” technique serves as a basis. This “Edge Motion Detection” technique essentially consists in a determination of the movement of edges (i.e. a difference between the intensity of pairs of pixels) in the image detected by the photodetector array. Edges are defined as spatial intensity differences between two pixels of the photodetector array. The relative motion of each of these edges is tracked and measured so as to determine an overall displacement measurement which is representative of the relative movement between the photodetector array and the illuminated portion of the surface.
In optical sensing devices, it is commonly known to couple a conversion circuit (or integration circuit) to each photosensitive element of the photodetector device so as to integrate the output signals of these photosensitive elements over time during a so-called integration period. FIG. 1 schematically shows the general principle of an integrating circuit, designated by reference numeral 1100, coupled to a photosensitive element, in this case a photodiode, designated by reference numeral 1000. This integrating circuit 1100 typically consists of an amplifier 1110 and a capacitive element 1120 (or integration capacitor) connected between the output and the inverting input of the amplifier, the photosensitive element 1000 being connected to the inverting input of the amplifier while the non-inverting input of the amplifier is tied to a reference potential such as ground. The integrating circuit 1100 outputs a voltage signal Vout, or integrated signal, which varies over time and which is in essence the result of the integration over time of the current signal iout produced by the photosensitive element 1000. Assuming that current iout has a substantially constant value during the period where integrating circuit is active (i.e. during the so-called integration period), the output voltage Vout will vary substantially linearly over time. In some cases, the integration period is set to have a fixed duration. In some other cases, however, the duration of the integration period may be variable.
Such optical pointing devices are already known in the art. U.S. Pat. No. 6,963,059, filed in the name of the same Assignee and which is incorporated in its entirety herein by reference, for instance discloses a method, and system for optimizing illumination power and integration time in an optical sensing device.
FIG. 2 illustrates the basic principle of U.S. Pat. No. 6,963,059. It basically consists of an optical sensing system comprising a light source 10 for illuminating a portion of a surface S with radiation, a photodetector device 20 having at least one photosensitive element responsive to radiation reflected from the illuminated surface portion S, conversion means 30, coupled to the output of the photodetector device 20, for integrating an output signal of the said at least one photosensitive element over time during an integration period of variable duration and a regulating system 40 for controlling the power of the light source as a function of the duration of the integration period. It should be stressed that the optical sensing system is designed, so that the integration period has a variable duration, designated Tint, which depends on the power of the light source 10 and the level of radiation reflected from the illuminated surface portion S. It will thus be appreciated that the optical sensing system of FIG. 2 includes some sort of feedback loop for enslaving the power of the light source 10 as a function of the evolution of the integration. The regulating system 40 is used to control (i.e. adjust if necessary) the power of the light source so that the duration of the integration period remains, under normal conditions, in the vicinity of at least one reference duration value. As schematically illustrated in the example of FIG. 2, three reference values designated Tmin, Tmax and Ttimeout may be used.
These optical pointing devices are more and more cordless devices, i.e. battery powered, and used on a wide variety of surfaces. Thus, for these optical pointing devices two very important features are surface coverage and tracking quality along with battery life. Darker surfaces require more light and thus a longer amount of integration time or a more powerful light source to recover the same amount of light energy as lighter surfaces. This longer integration time of the recovered light or this more powerful light source respectively causes the supported speed of motion to drop proportionally or the power consumption to increase proportionally. One way to optimize this behaviour is to stop the integration of light sooner on dark surfaces. Unfortunately this also decreases the signal to noise ratio (SNR) generally resulting in poorer tracking.